This invention relates to pulleys and more particularly to pulleys having integral hubs that are formed from sheet metal blanks. There are a variety of techniques employed in the fabrication of pulleys having hubs. One such technique involves spin forming the web and rim from a sheet metal blank, machine forming a hub, and attaching the hub to the web by welding or brazing. While this technique facilitates the formation of a hub having a complex shape, the separate operations involved add significant costs to the pulley. More recent techniques involve the radial displacement of metal from a sheet metal disc to integrally form a hub having a blind bore therein. Such a technique is described in U.S. Pat. No. 5,987,952. According to that patent, a shaping roller is pressed against a face of a spinning annular disc of sheet metal while the disc is supported at its opposite face with a rotating head stock mandrel. The shaping roller is moved progressively radially inwardly against the face of the rotating disc to displace a portion of metal in the form of a traveling annular wave while thinning the disc. The annular wave is pressed against a rotating, axially extending stepped mandrel with an end sized to press and hold an annular inward portion of the disc. The wave, which is pressed against the mandrel, forms the hub and the annular inward portion held by the mandrel becomes a blind bore for the hub.
A disadvantage of a blind bore formed in accordance with the aforementioned patent is that its location in the bore will not position a bearing close to an ideal location within the hub. It is desirable for the bearing to be located near a centered location in the hub for uniform loading of the bearing. It is desirable, therefore, to produce a hub in which the blind bore provides a stop at a location that is spaced from the web end a distance greater than the thickness of the original sheet metal.
This invention provides a method of forming a blind bore hub integrally with a circular disc of sheet stock, wherein the blind bore acts as a stop within the hub at a location that is spaced a greater distance from the web end of the hub than the thickness of the original sheet stock.
According to the invention, an annular disc is provided having an initial thickness and a center hole diameter substantially corresponding to the inside diameter of the hub to be formed. The disc is placed on a head stock mandrel and is spun while a shaping roller engages the disc and is moved progressively radially inwardly from a position about halfway between the edge of the disc and the center of the disc. The roller is also moved downwardly at a slight angle to move metal toward the center of the disc. A stepped mandrel is positioned in the hole of the disc with a small diameter portion extending through the hole and a larger diameter portion being positioned above the top face of the disc. As the roller moves toward the center of the disc, metal is forced into contact with the smaller diameter portion of the mandrel and upwardly toward the larger diameter portion of the mandrel to define a reduced diameter hole in the disc having a periphery which is above the plane of the disc face.
After this initial operation, the roller is backed off to a position proximate the rim of the disc and is moved progressively radially inwardly against the face of the rotating disc to displace a portion of metal in the form of a traveling annular wave while thinning the disc. The annular wave is pressed against the larger diameter portion of the rotating mandrel to form the hub and the previously formed metal which surrounds the smaller diameter portion of the mandrel forms a blind bore within the hub.